The Allosaurus was a large carnivorous dinosaur that walked on two legs and had two arms on the front of its body. This dinosaur could grow to be 12 meters long (38 feet) 5 meters tall (16.5 feet) and weigh up to about 1.4 tons (3086 lbs)! This dinosaur had a very long head, 90cm (3 ft), with many 5 to 10 cm teeth. The Allosaurus did not look like the other dinosaurs that existed at that time. Because of this, the word Allosaurus means "different lizard".

The Allosaurus is in the theropod (meaning "beast foot") family. Allosaurus had two arms. Its arms were longer than those of the T.Rex with large wicked claws. Allosaurus was bipedal.

Allosaurus had 2 crests above its eyes which may have been used for mating display.

Allosaurus were carnivorous dinosaurs. Allosaurus hunted in small groups which allowed them to take down even the largest prey. In the same way as raptors would, only slightly smaller groups. They even ate the gigantic Apatosaurus (which used to be called Brontosaurus) even though the Apatosaurus was more than three times the size of an Allosaurus. The Allosaurus was also likely to have hunted Stegosaurus, and Iguanodonts such as Camptosaurus.

Young baby Allosaurus might have eaten insects like dragonflies and centipedes, and other small animals. When about two years old, Allosaurus might have eaten small dinosaurs (Othnielia, Dryosaurs and so on).

Allosaurs competed for food with ceratosaurs when they first arrived but may have eventually driven the ceratosaurs to extinction. Allosaurus's teeth were thin and serrated not made to puncture bone but to strip bits of meat off a carcase. Often Allosaurus left pieces of meat on the bone that other predators like Ornithelestes would scavenge.

Like all dinosaurs, Allosaurus lived in the Mesozoic era. Mezo means middle so Allosaurus lived in the middle era. They lived in that era's late Jurassic period, between about 154 and 115 million years ago.

Allosaurus was a very common dinosaur around 135 million years ago. It lived on the land that now makes up the Morrison Formation, USA and Portugal. A major discovery was made in Colorado when a rancher named M. P. Felch discovered an almost perfect skeleton of an Allosaurus. They have also been found in Africa, which was connected to North America by a land bridge at the time. There is a site in Utah where the fossils of more than 40 Allosaurus have been found. Some close relatives lived in Asia.

Allosaurus probably lived on the plains and lowlands where groups could hunt easily. It probably had to compete for food against Ceratosaurus and some species of Dilophosaurus. Some allosaurs lived in Europe. There is proof that Allosaurus lived in wetlands. One find in Utah shows that a "predator trap" may have made plant eating dinosaurs trapped in sticky mud, and the allosaurs wanted an easy snack, but they soon were caught, and died one by one.

The first fossil of an Allosaurus to be discovered was found in the USA state of Colorado, in 1869. At first, the people who found it thought it was a petrified horse hoof instead of a dinosaur bone. In 1991 a complete Allosaurus skeleton was discovered. This Allosaurus was named "Big Al". When scientists looked at the skeleton they discovered that "Big Al" was actually an Allosaurus teenager. They also discovered that this young Allosaurus was about 8 meters (26 feet) long! "Big Al" was made famous by a TV programme called The Ballad of Big Al also called Walking with Dinosaurs: Allosaurus. This program showed what Big Al's life could have been like. They are also one of the most common dinosaurs in museums from the Jurassic time period.

How big their hunting groups were. It isn't even certain that they did hunt in groups, although until recently, most people assumed that they did. They did not hunt in groups or they were really, really, rare. Also if one came across another they would fight until they either saw limping or blood. They did not see each other as enemies; it was just natural instinct. Although they hunted dinosaurs that lived in groups they did not or they were like one in a million rare of hunting in groups.

How much did they eat each day? They ate about 20 pounds per meal so 60 pounds a day.